The present invention generally relates to high speed generators and, more specifically, to apparatus and methods for regulating voltage to a DC power distribution bus over a wide speed range in a high reactance permanent magnet machine based electrical power generation system.
Electrical power generation systems (PGS) play a significant role in the modern aerospace/military industry. Recently, this is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft. The commercial aircraft business is moving toward no-bleed air environmental control systems (ECS), variable-frequency (VF) power distribution systems, and electrical actuation.
Military ground vehicles have migrated toward hybrid electric technology, where the main propulsion is performed by electric drives. Therefore, substantial demand for increased power generation has emerged. Future space vehicles will require electric power generation systems for thrust vector and flight control actuation. These systems must be more robust and offer greatly reduced operating costs and safety compared to the existing Space Shuttle power systems.
These new aerospace trends have significantly increased power generation needs. This has led to increased operating voltages to reduce system losses, weight, and volume. New power, quality and electromagnetic interference (EMI) requirements have been created to satisfy both quality and performance needs. The overall result has been a significant increase in the installed electric power, creating challenges in accommodating this equipment in the new platforms. Therefore, overall system performance improvement and power density increases are necessary for the new-generation hardware to satisfy MEA. Decreasing the cost of power generation systems will make the new platforms more affordable.
Wide Speed Range (WSR) PGS applicable to MEA must satisfy a complex set of requirements. The main function of such a system is electrical power generation; hence the system must provide conversion of the mechanical power supplied by the prime mover to conditioned electrical power supplied to the distribution bus. Generation is typically defined as continuous power at 100 percent load. Increasing the load to 150 percent for a limited time may be required. The percentage of increase and time required for overloading varies from application to application.
Another requirement for WSR PGS applicable to MEA is steady-state regulation, which requires that the system maintain the output voltage constant within certain limits when the loads and other conditions are changed gradually. Transient regulation is a requirement that the system maintains the output voltage constant within certain limits when the loads and other conditions are changed rapidly. Transient limits are typically wider than steady-state limits. Typical regulation requirements can be found in MIL-STD-704E. Electromagnetic interference (EMI), both conducted and radiated emissions, are important requirements for an EPGS to provide proper operation of the installed electronics. At the same time, the electronic equipment including PGS should not be susceptible to the specified radiated emissions.
DC bus short-circuit protection is another requirement which must provide adequate protection when an external short-circuit fault occurs at the DC distribution bus. Feeder short-circuit protection function is also required to prevent excessive current flow in the electric machine and the interface electric machine power electronics to reduce damages that may lead to a hazardous condition. Power electronics short-circuit protection is required to prevent excessive current flow in the power electronics unit. Overvoltage protection is required to prevent excessive voltage across a power distribution bus. Overvoltage protection prevents damage of the electronics connected to the distribution bus.
Electric machines used in auxiliary power unit (APU) applications typically operate at constant speed or with small variation. The main engines of an airplane normally operate with a speed range where the ratio of maximum to minimum operating speed is about 2 to 1. This speed variation creates additional difficulties for a power generation system in providing regulated power within the entire speed range. There are some applications where the speed of the prime mover, for instance a helicopter engine, changes by a factor of up to 20. This wide speed range creates even more challenges due to variation of the electromotive force (emf) voltage of the machine with the speed.
The synchronous permanent magnet machine (PMM) presents a very competitive design that outperforms other electric machines in most applications when weight and size are critical. However, the rotor flux in a typical PMM is fixed and cannot be controlled or disengaged when a short-circuit is initiated. Unlike other machines where the excitation of the rotor flux can be controlled and even disabled quickly, a PMM continues to generate emf until the rotor stops rotating. Therefore, the PMM presents a hazard in some applications, leading to its limited use, particularly in the aerospace industry.
The High Reactance Permanent Magnet Machine (HRPMM) is one type of PMM in which, should it become shorted, the phase current magnitude can be internally limited to levels capable of being sustained either indefinitely, within the thermal limits of the system, or until the rotor speed can be reduced to zero. In some prior HRPMM power topologies the functional and protection requirements may be resolved. However, the operating speed range may still be quite narrow.
As can be seen, there is a need for a PMM-based power generation systems that can supply power to a DC bus within a wide speed variation while satisfying the functional and safety requirements discussed above.